1. Field of the Invention
This invention generally relates to apparatus for dispersing fluids and more specifically to a method and apparatus for atomizing liquids.
2. Description of Related Art
Nozzles are used to atomize fluids, such as liquids, gases and liquid-solid slurries, to improve certain characteristics of the fluid. For example, nozzles for internal combustion engines and jet engines atomize fuel to produce fine fuel particles and improve combustion efficiency. Chemical processes use nozzles to atomize materials, such as water, to improve subsequent chemical reactions, such as aeration. Apparatus providing surface treatment or cleaning, such as high pressure washers, may use atomized water or cleaning solutions to impinge material to be cleaned with some momentum to improve surface washing.
In each of these applications, it is often desirable to reduce droplet size to the smallest possible dimension. For combustion and chemically processing, reducing droplet size maximizes the surface area per mass unit that is available for combustion or other chemical reactions. With surface treatment and cleaning, the reduction in size increases the number of drops that impact the surface under treatment. It is also desirable in many of these applications to maximize flow rates through the nozzle usually by increasing the pressure applied to the liquid at the nozzle input.
A number of diverse nozzle constructions have evolved for atomizing liquids. Garden hose and fire hose nozzles are well known examples. Other nozzles atomize fuel, in liquid or slurry form, to increase combustion efficiency. Whether such nozzles do, in fact, increase combustion efficiency is readily determined by examining decreases in fuel consumption for a given thermal output and the nature and quantity of combustion by-products.
For example, nozzles used in tube furnaces have been fitted with spirally shaped narrowing tips that direct atomized fuel along a spiral flow path. This flow path increases the time required for the fuel to pass through an area of combustion or through a flame zone. Although this leads to more complete combustion, this nozzle also produces large droplets. Unburnt fuel precipitates as a solid combustion product. While this approach provides some improvement, incomplete combustion still occurs due to less than optimal atomization.
In an approach described in USSR Inventor Certificate No. 657,858, a spray-swirl atomizer receives liquid under pressure at a nozzle input in a stream form. The liquid stream passes through a narrowing opening. The nozzle swirls the liquid stream prior to ejection through an exit orifice. This nozzle comprises a casing, a supply pipe and a swirler with half cylinder channels directed at about 25.degree. to the axis of the apparatus. Swirling the liquid produces a centrifuging effect that breaks the liquid into droplets as it emerges from the nozzle. However, this structure tends to concentrate the droplets in a conical volume that is displaced from the axis of the nozzle. Thus the liquid exits the nozzle in an atomized form confined to a conical volume or tongue with a central conical volume along the nozzle axis that is substantially devoid of any fuel. Apparatus in the form of an air delivery system increases the atmospheric pressure acting about the exit of the nozzle in order to effect a transfer of the atomized liquid toward the axis and fill the volume. These nozzles do not produce fine atomization and have been characterized by the formation of back currents that detract from the effectiveness and quality of atomization. Moreover, the nozzle channel dimensions are selected to prevent the existence of conditions that would allow the formation of cavities in the liquid.
Nozzles normally atomize fluids by passing a liquid from a supply passageway through a small diameter passage or orifice at an increased velocity. As higher and higher pressures are applied to the input passageway to increase liquid flow and velocity, the static pressure of the liquid decreases. More specifically, the potential energy represented by the static pressure transfers into increased liquid momentum or kinetic energy. Stated differently, the total potential and kinetic energy of the liquid remains constant as the liquid passes through the nozzle assuming that no external energy is applied to the liquid. If P.sub..infin. represents the static pressure of the liquid (i.e., the potential energy component) and if g.sub.o represents the gravitational acceleration and V.sub..infin. represents the velocity of the liquid that define the kinetic energy component, then Bernoulli's theorem can be written as: ##EQU1## where K is a constant.
Such liquids often carry dissolved or entrained gases having their own internal or partial pressures. If the velocity of the liquid, V.sub..infin., increases and static pressure, P.sub..infin., decreases, the partial pressure of the entrained gas remains relatively constant. Consequently, the gas molecules tend to expand and produce bubbles or cavities within the liquid. If the liquid velocity and input pressure exceed certain minimums, this cavity production, or nucleation, becomes significant.
As liquid leaves the orifice, its velocity, V.sub..infin., decreases rapidly. Consequently the kinetic energy of the liquid due to momentum decreases and the potential energy of the liquid represented by its static pressure, P.sub..infin., increases. Eventually the liquid static pressure reaches a threshold that prevents further cavity growth and actually acts to compress the cavities and collapse them. This collapse produces high bubble wall velocities, high temperatures and large shock forces.
This collapse occurs inside prior art nozzles. The resulting large shock forces can erode the interior of a nozzle and eventually destroy its effectiveness. This phenomenon of bubble formation and collapse therefore has been a limiting factor in the design of prior art atomizers.
Hammitt, Cavitation and Multi-Phase Flow Phenomena, McGraw-Hill, Inc. New York 1980, discusses this phenomenon in detail. In one specific example, Hammitt discloses a Venturi tube with an entrance cone that converges to a constant diameter throat of diameter d.sub.n and an exit cone or diffusion section. The exit cone defines a right conical surface that expands along a straight line at a constant angle with respect to the axis through the Venturi tube. When this nozzle operates under conditions that would allow cavitation, erosion can be observed at several locations within the conical diffusion section. In one particular example, metal erosion occurs in the exit cone at 3d.sub.n, 6.5d.sub.n and 10d.sub.n downstream from the throat-conical diffusion section interface.
This prior art understanding of cavitation led to a nozzle design philosophy under which nozzle input pressure and liquid velocity are increased to a point just below which significant cavity nucleation begins. Stated differently, these prior art designs avoid or minimize the forces produced by cavity collapse by preventing cavitation or limiting cavitation to insignificant levels. The prior art has never suggested any positive purpose or use for these forces other than as part of flow rate measurement or flow limiting techniques and apparatus.
Other prior efforts have been directed to methods of activating cavitation in liquids for various purposes. For example, U.S.S.R. Inventor Certificate No. 1,227,000 (1984) describes the application of ultrasonic energy to liquids in capillaries of 0.02 to 5.0 mm diameter in order to increase the gas content of a liquid. One outcome of this effort was to determine that a gas concentration of 1.5% was critical to the formation of cavities in a liquid. None of this work, however, was directed to controlling cavity collapse. The application of ultrasonic energy to the liquid apparently only assisted in the mixing of the gas and the liquid. Consequently this effort was directed to increasing the density of gas bubbles or cavities in the liquid. However, nothing was suggested with respect to controlling cavity collapse.
It has also been suggested to excite cavitation by swirling a liquid stream about an axis with subsequent convergence of the stream and ejection of the converged stream through an exit orifice Kerimov et al., "Increase in the Efficiency of Burning Residual Fuel Oil by Using Ultrasonic Atomizers", For Technical Progress, No. 8, Page 25 (1978). This disclosure anticipates that cavities will form, expand and collapse thereby to produce ultrasonic waves that will atomize the liquid. In practice this process yields only a low level or intensity of cavitation. Moreover, the cavity collapse, albeit at low levels, occurs within the nozzle. Thus, even if the intensity of the cavity collapse were to increase to useful or practical levels, the collapse would still occur within the nozzle and erode it.
My U.S.S.R. Inventor Certificate No. 1,708,436 describes a nozzle that attempts to harness the forces produced by cavity collapse by forestalling the development of these forces until the cavities exit the nozzle. Like the previously mentioned prior art, liquid with an entrained gas passes through a swirler. Unlike the prior art, a low molecular gas or vapor, typically steam, is added to the liquid in an amount not less than 10.sup.-3 to 10.sup.-2 fractions of the total mass. The steam mixes upstream of a swirler at a distance not less than 10 times the diameter of the exit orifice of the atomizer. As the stream with the introduced gas or vapors swirls along a spiral, convergent path about the nozzle axis, the added gas or vapor particles shift toward the nozzle axis. That is, the steam cavities or bubbles essentially centrifuge and move to the center. This permits the gas concentration along the nozzle axis to increase. These gas particles become cavitation centers and continue to be produced until the static pressure component in the surrounding liquid stream reaches the pressure of saturation of the liquid vapors. A finite time occurs between the emergence of the liquid from the swirlers and the time the cavities collapse for a given rise in static liquid pressure. Controlling the velocity can localize the zone of cavitation and enable each of the cavities or bubbles to store energy in the order of 50 kcal/kg. However, if the pressure is allowed to increase inside the nozzle, cavity collapse can occur inside the nozzle and disrupt the flow from the swirlers thereby causing additional collapse within the nozzle and result in nozzle erosion.
Nozzles constructed in accordance with these later approaches have shifted the zone of cavity collapse to a site outside the nozzle. However, the effectiveness of this control has been dependent upon the velocity of the stream or the ability to operate the nozzle without having the internal pressures of the environment surrounding the nozzle or the static pressure of the liquid rise to a level that would induce cavity collapse. The collapse and attendant energy release have produced droplet sizes in the order of 50 .mu.m. Yet the full useful potential of cavity collapse has not been fully realized in any of these prior art nozzles. Nozzles continue to produce droplet sizes of larger than optimal size particularly as evidenced by the production of undesirable combustion by-products when fuel oil is burned.